Transition State Characterization of the Low- to Physiological-Temperature Nondenaturational Conformational Change in Bovine Adenosine Deaminase by Slow Scan Rate Differential Scanning Calorimetry

Bovine adenosine deaminase undergoes a nondenaturational conformational change at 29 degrees C upon heating which is characterized by a large increase in heat capacity. We have determined the transition state thermodynamics of the conformational change using a novel application of differential scanning calorimetry (DSC) which employs very slow scan rates. DSC scans at the conventional, and arbitrary, scan rate of 1 degree C/min show no evidence of the transition. Scan rates from 0.030 to 0.20 degrees C/min reveal the transition indicating it is under kinetic control. The transition temperature T(t) and the transition temperature interval deltaT increase with scan rate. A first order rate constant k1 is calculated at each T(t) from k1 = r(scan)/deltaT, where r(scan) is the scan rate, and an Arrhenius plot is constructed. Standard transition state analysis reveals an activation free energy deltaG(double dagger) of 88.1 kJ/mole and suggests that the conformational change has an unfolding quality that appears to be on the direct path to the physiological-temperature conformer.